Disc drive data handling devices are commonly used as the primary data storage and retrieval devices in modem computer systems. In a typical disc drive, data are magnetically stored on one or more discs that are rotated at a constant high speed and accessed by a rotary actuator assembly having a plurality of read/write heads that fly adjacent the surfaces of the discs. A read channel and interface circuit are provided to recover previously stored data from the discs to the host computer.
A digital servo system is typically used to control the position of the heads relative to tracks on the discs. The tracks are defined from servo data written to the surfaces of the discs during manufacturing. The servo system utilizes the servo data in the performance of two primary operations: seeking and track following.
Seeking entails the movement of a selected head from an initial track to a destination track. Current is applied to an actuator coil to initially accelerate and then decelerate the head to bring the head to rest upon the destination track. In one approach, the velocity of the head is repetitively determined and compared to a velocity profile which defines an optimum velocity trajectory for the head as it moves to the target track. The amount of current applied to an actuator coil varies in proportion to the velocity error. In another approach, current is applied directly to the coil in accordance with a predetermined current or source voltage profile appropriate for the length of the seek. In either case, once the head reaches the destination track the servo system transitions to a track-follow mode wherein the head is maintained over the destination track until the next seek command is executed.
Since user data are only typically transferred to the host device during track following modes of operation, efforts are made by disc drive designers to minimize the time required to carry out seek operations in order to maximize overall data transfer performance. Actuators are typically abruptly accelerated and decelerated to bring the head into alignment with the destination track in as short a time as practical. However, such abrupt forces upon the actuator can excite resonances of the mechanical actuator system, causing the head to oscillate (ring) upon arrival to the destination track. Such oscillations can significantly lengthen the time required to complete the seek and transition to the track following mode.
Designers have employed various techniques to select trajectory control profiles (velocity, current, etc.) having shapes that reduce the excitation of such resonances. One approach discussed by Mizoshita et al. in a paper entitled “Vibration Minimized Access Control for Disk Drives,” IEEE Transactions on Magnetics, Vol. 32, No. 3, May 1996, uses a minimum-jerk cost function to develop a closed-loop velocity profile to move a disc drive actuator arm. As will be recognized, jerk is defined as change in acceleration (force) with respect to time.
While operable, the system of Mizoshita has certain drawbacks, including inadequate compensation for certain real world effects such as coil inductance and back electromagnetic force (bemf) in the actuator coil. These factors mitigate against using the derived profiles in an open loop fashion; applying current from a voltage source in accordance with the derived current profile would not likely provide the necessary current within the coil to achieve the desired final states for the seek (correct final position, velocity and acceleration of zero, etc.). Using this methodology in a closed-loop fashion may also result in less than optimum performance; applying whatever current is necessary to cause the head to follow the derived velocity profile might provide different jerk characteristics than those predicted by the model due to the effects of bemf and coil inductance.
Mizoshita's cost function is expressed in terms of the integral of the square of the jerk profile values, and thus attempts to minimize the area of the jerk profile. While this tends to reduce excitation of all mechanical resonances of the system across the board, there is no provision to further address and suppress the excitation of specific resonant frequencies of the mechanical system identified as potentially having an adverse effect upon the operation of the servo system. Thus, even using Mizoshita's jerk profile such resonances can still be excited during a seek, increasing the time required to complete the settling of the head onto the destination track.
Accordingly, there is a continued need for improvements in the art to provide control profiles to facilitate positional control of a control object such as a disc drive actuator. It is to such improvements that the claimed invention is directed.